A photoelectric element is an element that can convert energy of photons into an electric signal via some physical phenomenon (photoelectric conversion). A solar cell is a kind of the photoelectric elements, and can effectively convert light energy of sunlight into electrical energy.
Known semiconductors to be used in the solar cells include single-crystal Si, polycrystalline Si, amorphous Si, GaAs, InP, CdTe, CuIn1-x GaxSe2 (CIGS), Cu2ZnSnS4(CZTS), and the like.
Among them, chalcogenide compounds, typified by CIGS or CZTS, have a large optical absorption coefficient, and thus enable formation of a thin film which is advantageous for reduction in cost. In particular, solar cells using CIGS in a light-absorbing layer show the highest conversion efficiency among thin film solar cells, and obtain excellent conversion efficiency as compared to solar cells using polycrystalline Si. The CIGS, however, has the problems of inclusion of environmental load elements and rare elements.
In contrast, CZTS has band gap energy appropriate for solar cells (of about 1.4 to 1.5 eV) free from environmental load elements and rare elements. Solar cells using the CZTS in a light-absorbing layer, however, have low conversion efficiency as compared to solar cells using a conventional semiconductor in a light-absorbing layer.
Various proposals have hitherto been made to solve the forgoing problems.
For example, a non-patent document 1 discloses a manufacturing method of a CZTS-type solar cell which involves:
(1) placing a Mo-coated soda lime glass (SLG) substrate in a vacuum chamber, and fabricating a precursor film on the substrate by co-sputtering using ZnS, SnS, and Cu as a target, and
(2) then transferring the substrate to an anneal chamber, introducing N2+H2S (20%) reaction gas into the anneal chamber, and annealing the substrate at 580° C. for three hours.
This document describes the following:
(a) when sputter powers are set to 160 W for ZnS, 100 W for SnS, and 95 W for Cu, a CZTS film is obtained which has a ratio of Zn/Sn=1.18, and a ratio of Cu/(Zn+Sn)=0.94, without a dependence on the thickness of the CZTS film; and
(b) when the Cu target power is decreased to 89 W, the film is obtained which has an open circuit voltage Voc=662 mV, a short circuit current density Isc=15.7 mA/cm2, a fill factor F.F.=0.55, a conversion efficiency=5.74%, a series resistance Rs=9.04Ω, a parallel resistance Rp=612Ω, Cu/(Zn+Sn) ratio=0.87, and Zn/Sn ratio=1.15.
A non-patent document 2 discloses a manufacturing method of a CZTS-type solar cell which involves:
(1) depositing a Cu/Sn/ZnS stacked precursor or a Sn/Cu/ZnS stacked precursor on a Mo-coated SLG substrate by using electron-beam evaporation technique, and
(2) sulfurizing the precursor in an atmosphere of H2S (5%)+N2.
This document describes the following:
(a) When the Cu/Sn/ZnS stacked precursor is formed on the substrate, the surface of a thin film obtained after the sulfurization is rough and many voids are found therein. In contrast, when the Sn/Cu/ZnS stacked precursor is formed on the substrate, no large voids are found on the surface of a thin film obtained after the sulfurization.(b) When the Cu/Sn/ZnS stacked precursor provided by stacking a ZnS layer of 330 nm in thickness, a Sn layer of 150 nm in thickness, and a Cu layer of 110 nm in thickness in that order over the substrate is sulfurized at 550° C., the film is obtained which has an open circuit voltage Voc=621 mV, a short circuit current density Isc=10.33 mA/cm2, a fill factor F.F.=0.55, a conversion efficiency=3.50%, a series resistance Rs=8.7Ω, a parallel resistance Rsh=302 n, Cu/(Zn+Sn) ratio=0.85, and Zn/Sn ratio=1.19.(c) When the Sn/Cu/ZnS stacked precursor provided by stacking a ZnS layer of 340 nm in thickness, a Cu layer of 120 nm in thickness, and a Sn layer of 160 nm in thickness in that order over the substrate is sulfurized at 520° C., the film is obtained which has an open circuit voltage Voc=629 mV, a short circuit current density Is=12.53 mA/cm2, a fill factor F.F.=0.58, a conversion efficiency=4.53%, a series resistance Rs=8.5 0, a parallel resistance Rsh=428Ω, Cu/(Zn+Sn) ratio=0.85, and Zn/Sn ratio 1.03.
Further, a non-patent document 3 discloses a CZTS-based sulfide having a Cu/(Zn+Sn) ratio=0.96 and a Zn/Sn ratio=1.08.